ISO 50?

I'm out shooting the ocean waves and rocks and playing around with filters and stacking. I start to lose the ability to auto focus and I notice some vignetting. So I started to think why is ISO 50/100 the lowested.

My question for techies, is what makes it so hard to get a sensor to go lower than 50? We found ways to impove high ISO and continue to go up and up. Why is 50/100 still after 50 years the lowest why have availible to us? Peace!